This invention is an improved machine that can be used to reorder or dry tobacco, or treat other particulate solid materials, e.g., food. The apparatus comprises a self-stacking spiral conveyor that traverses through a circulating atmosphere, which is manipulated by one or more chambers having an open side adjacent to the perforated sides of the spiral conveyor.
The tobacco art has long recognized the desirability of controlling the moisture content of tobacco. Tobacco is processed in a series of steps with each step requiring its own optimal tobacco moisture level in order to produce the highest quality tobacco product. The moisture content of tobacco that has been processed into a useful product is thus typically altered numerous times. The process by which the moisture content of the tobacco is altered can have a lasting effect on the physical, chemical, and subjective characteristics of the final product. The risk of damage to the tobacco while changing the moisture content (e.g., drying or reordering) make control of the process and uniformity of treatment very important. To accomplish product uniformity, a gas to solid treatment apparatus is necessary that treats the tobacco in a uniform manner.
A gas treatment apparatus in the form of a self-stacking spiral conveyor is known where products on the conveyor are treated by heat transfer from a treatment gas, such as refrigerating, freezing, or heating while the products are transported on the conveyor through the treatment gas. The conditioning means for the treatment gas may be a heat exchanger which cools or heats the treatment gas before it circulates around and through the bed of conveyed product.
For example, a Frigoscandia self-stacking spiral conveying machine (e.g., a Model GCP 42 self-stacking spiral freezer supplied by Frigoscandia Food Process Systems AB of Helsingborg, Sweden) is disclosed in U.S. Pat. Nos. 3,938,651, 4,565,282, 4,603,776, 4,899,871, and 4,941,567. This apparatus, by virtue of its self-stacking spiral design and gas recirculation fans, channels the majority of gas flow downward through the multiple tiers of the open mesh conveyor, which carry the product.
By feeding product into the bottom of the conveyor stack, which is formed by the multiple tiers of the conveyor, and forcing air into the top of the conveyor stack, the overall flow of air and product is essentially countercurrent. The Frigoscandia device is well adapted for thermal treatment of discrete items that are conveyed by an open mesh conveyor belt that offers very little resistance to gas flow in the vertical direction. The present invention discloses an improvement over the known apparatus with the improvement allowing the machine to perform new tasks--including treating a packed bed of conveyed product to alter its moisture content or otherwise chemically or thermally treat the packed bed. Although the present invention is applicable to the removal of moisture and other chemicals from the conveyed material, the illustrative description will address only the addition of moisture or other chemicals from a high concentration treatment gas.
The discussion of the present invention will benefit from a description of the four different types of gas flow within the typical self-stacking spiral conveying apparatus.
(1) Vertical flow--gas flow through the openings in the bottom member of the conveyor links and the bed of conveyed product in an upward or downward direction;
(2) Radial flow--flow transverse to the conveyor belt links through perforations in the walls. Perforations include the gaps between successive conveyor belt links, the gaps between successive conveyor stack tiers, and the holes in the spacer members that make up the side walls of the conveyor stack, if such holes have been added to increase radial flow. Radial flow includes both radially inward and radially outward flows;
(3) Spiral flow--flow along the spiral trough formed by the self-stacking spiral conveyor belt. Spiral flow is influenced by among other factors, the clearance between the top of the bed of conveyed material and the bottom of the bottom element of the conveyor belt lying on top of the side walls; and
(4) Bypass flow--flow along the exterior walls of the cylinder formed by the conveyor stack. The term bypass flow extends to all flow of gas outside of a flow path through the conveyor stack.
The present invention modifies a known device, such that several parameters of the gas within the device, including relative humidity, pressure temperature, velocity, flow direction, and concentration of any additive can be effectively controlled so as to provide control over the treatment given to the solid material on the conveyor.
The known Frigoscandia device is designed to give the proper gas flow pattern to thermally treat discrete items such as hamburger patties on a open mesh belt. Difficulty arose in part from the difference in the resistance to gas flow by the packed bed application.
Vertical gas flow is highly desirable because it treats the interior of the packed bed of conveyed material, instead of merely the material on the surface of the packed bed.
The amount of spiral flow could be controlled by the clearance between the packed bed of conveyed materials and the next tier of endless conveyor belt. The clearance could be controlled by adjusting the depth of the packed bed relative to the height of the side spacers. A second method of reducing the spiral flow would use upstanding flanges attached to the foraminous bottom member, the flanges being positioned at least partially transverse to the direction of belt travel.
Bypass flow fails to contact the product and is inherently not effective. Bypass flow is also a problem because treatment gas may enter the stack at the wrong point. For example, bypass flow may bypass upper tiers of the conveyor stack and then enter a lower tier of the conveyor stack to contact the bed of conveyed material. When this occurs, the gas in the bypass flow may have a more concentrated level of conditioning agent than is desired at that tier of the conveyor stack because the bypass flow did not previously contact all of the upper tiers of the bed of conveyed product to transfer the conditioning agent to the conveyed product. This exposure of conveyed product to treatment gas having elevated levels of conditioning agent, in a process where the material being processed follows a predetermined set of treatment conditions, may cause product damage, because the conveyed material may be intolerant to a deviation from the preset treatment conditions. For example, copending, commonly-assigned U.S. patent application Ser. No. 07/969,109 (PM-1612(II)), filed concurrently herewith and hereby incorporated by reference in its entirety, discloses processes for reordering and drying tobacco, teaches that conditioning air having a high relative humidity for use in the final stages of reordering tobacco is potentially damaging to tobacco in an earlier stage of reordering. This requirement of strictly segregating conditioning gas that is tolerated by the conveyed product only in the final stages of treatment from conveyed product entering treatment was not a problem faced by the known Frigoscandia device in its application of cooling products on an open mesh belt.
Thus, bypass flow may makes the prior art apparatus inefficient if used without modification to perform this new function of non-thermal treatments of a packed bed. One potential source of inefficiency related to bypass flow is the additional energy requirements to circulate and condition any additional treatment gas. While U.S. Pat. Nos. 4,089,666 and 4,178,946 describe improved processes for conditioning the air to specific conditions of temperature and relative humidity, the process remains energy intensive. The cost to treat the gas to a specific inlet condition is not a problem for the known Frigoscandia device when used in thermal treatments of products since the thermal energy in the small amounts of bypass flow in the open mesh thermal treatment conveyor process is recycled without loss in the conditioning means.
Thus, improved control of bypass flow is desirable especially for processes where the conveyed material is susceptible to damage from treatment gas that deviates from the prescribed treatment ramp. Therefore, it is highly desirable to have a device that treats a bed of conveyed product in a substantially uniform manner, thus resulting in a homogeneous, fully-treated product without damage from localized pockets of over-treatment. The present invention increases the uniformity of conveyed product treatment at commercial conveyor speeds and reduces wasteful recirculation of treatment gas.